1. Field of the Invention
The present invention relates to a semiconductor device and also to a wire bonding method which connects first and second bonding points by a wire, and in particular, to a wire loop formation method.
2. Prior Art
The manufacture of a semiconductor device includes a process in which, as shown in FIGS. 3(a) and 3(b), a pad 2a (first bonding point) on a semiconductor chip 2 mounted on a lead frame 1 and a lead 1a (second bonding point) on the lead frame 1 are connected by a bonding wire (called merely "wire") 3. The loop shape of the wire 3 in this case may be a trapezoidal loop shape as shown in FIG. 3(a) or a triangular loop shape as shown in FIG. 3(b).
Wire loop formation methods of this type are described in, for example, Japanese Patent Application Publication (Kokoku) No. H5-60657 and Japanese Patent Application Laid-Open (Kokai) No. H4-318943.
The trapezoidal loop shape of FIG. 3(a) is formed by the process shown in FIG. 4.
In step (a), a capillary 4 is lowered so that a ball (not shown) formed on the tip end of the wire 3 that passes through the capillary 4 is bonded to a first bonding point A. This is done while a damper (not shown), which is located above the capillary 4 and can hold the wire when closed and feed the wire when opened, is opened. After the bonding to point A, the capillary 4 is raised to point B, delivering the wire 3.
Next, as seen in step (b), the capillary is moved horizontally in the opposite direction from the second bonding point G to point C. Generally, to move the capillary 4 in the direction opposite from the second bonding point G (for forming a loop in the wire) is referred to as a "reverse operation". Because of this reserve operation, the wire 3 assumes a shape that extends from point A to point C; and as a result, a kink 3a is formed in a portion of the wire 3. The wire 3 delivered in the process from point A to point C forms the neck height portion 31 of the loop shown in FIG. 3(a).
Next, in step (c), the capillary 4 is raised to point D while delivering the wire 3.
Afterward, as shown in step (d), the capillary 4 is again moved horizontally to point E in the opposite direction from the second bonding point G, i.e., another (or second) reverse operation is performed. As a result, the wire 3 assumes a shape inclined from point C to point E, and a kink 3b is formed in a portion of the wire 3. The wire 3 delivered out of the capillary 4 in the process from point C to point E forms the trapezoidal length portion 32 shown in FIG. 3(a).
Furthermore, in step (e), the capillary 4 is raised to point F while delivering the wire 3. The amount of wire 3 delivered is equal to the inclined portion 33 shown in FIG. 3(a). Afterward, the damper (again, not shown) is closed. Once the damper is closed, the wire 3 is not delivered even if the capillary 4 subsequently is moved.
As shown in steps (f) and (g), the capillary 4 performs a circular-arc motion (or a circular-arc motion followed by a straightly lowering motion) so that the capillary 4 is positioned at the second bonding point G, and the wire 3 is bonded to the second bonding point G, thus connecting the first and second bonding points A and G.
On the other hand, the triangular loop shape of FIG. 3(b) is formed by the process shown in FIG. 5.
In this triangular loop formation, the trapezoidal length portion 32 described in the trapezoidal loop formation process is not formed. Accordingly, the second reverse operation in step (d) in FIG. 4 is not performed. Thus, the steps (c), (d) and (e) in FIG. 4 are replaced by the single process as shown in step (c) of FIG. 4. In other words, the steps (a) and (b) are the same as the steps (a) and (b) shown in FIG. 5, respectively; and after the first reverse operation in step (b) of FIG. 5, the capillary 4 is raised to point F while delivering the wire 3 in step (c). Afterward, the capillary 4 performs the steps (d) and (e) in the same manner as the operations done in the steps (f) and (g) shown in FIG. 4, and the wire 3 is bonded to the second bonding point G.
As seen from the above, the triangular loop formation shown in FIG. 5 is simpler than the trapezoidal loop formation shown in FIG. 4 and is therefore advantageous in that the loop formation is performed in a shorter time. However, in cases where the height difference between the first bonding point A and the second bonding point G is large, or if there is a large distance between the first bonding point A and the edge portion of the semiconductor chip 2, then the wire 3 of the triangular wire loop shape as shown in FIG. 3(b) tends to come into contact with the edge portion of the semiconductor chip 2. In such cases, the trapezoidal wire loop formation is employed so that the contact between the wire 3 and semiconductor chip 2 can be avoided.
In the trapezoidal loop formation process shown in FIG. 4, the first reverse operation shown in step (b) is performed with the capillary 4 at a height close to the height of the first bonding point A. Accordingly, the kink 3a is relatively strong. However, the second reverse operation shown in step (d) is performed with the capillary 4 in a high position which is vertically far away from the first bonding point A. Accordingly, the kink 3b is difficult to form and is unstable. As a result, the portion of the wire near the kink 3b (see FIG. 3(a)) tends to be unstable and has a weak shape-retaining strength; as a result, this portion of the wire near the kink 3b may rise up or drop down. If the shape-retaining strength of the portion of the wire near the kink 3b is weak, then the wire is likely to bend when pressure from the outside is applied thereon. For instance, wire bending may easily occur when the capillary comes into contact with the second bonding point or when the impact by the ultrasonic oscillation is applied on the second bonding point. Such a wire bending can also occur when the wire vibrates or when the molding material flows during the process of injection of a molding material.
However, in the trapezoidal loop shape shown in FIG. 3(a) as well, as shown in FIG. 6, the following problem has been encountered if the height H of the loop is low, e.g., approximately 250 microns or less, and if the loop has a long wire length, i.e., in cases where (for example) the distance L between the first bonding point A and the second bonding point G is approximately 5 mm.
In particular, FIG. 7 shows a conventional trapezoidal loop formation process. The steps (a) through (e) correspond respectively to the steps (a) through (e) in FIG. 4. As shown in step (a), the capillary 4 is lowered so that the ball formed on the tip end of the wire is bonded to the first bonding point A, after which the capillary 4 is raised to point B and the wire 3 is delivered. Next, as shown in step (b), the capillary 4 is moved horizontally to point C in the opposite direction from the second bonding point G; in other words, a reverse operation is performed in step (b). As a result, a kink 3a is formed in the wire 3.
Next, as shown in step (c), the capillary 4 is raised to point D1, delivering the wire 3. Afterward, in step (d), the capillary 4 is again moved horizontally by a slight amount to point E1 in the opposite direction from the second bonding point G; in other words, another reverse operation is performed. As a result, a kink 3b is formed in the wire 3.
In cases where the wire length is short as shown in step (a) in FIG. 3, then the amount of wire 3 delivered in the step (c) in FIG. 4 may also be short. Accordingly, the kink 3b (see step (e) in FIG. 4) is formed by the reverse operation shown in step (d) in FIG. 4. However, in cases where the wire length is long as shown in FIG. 6, the amount of wire 3 delivered in step (c) in FIG. 7 is long. As a result, when the reverse operation shown in step (d) is performed, an excess amount of wire is delivered as a result of the tension on the wire 3 in the direction of the first bonding point A and the weight of the wire 3 itself. Accordingly, a bow 34 is formed in the wire 3, and the shape of the kink 3b is rounded, or no kink 3b is formed, resulting in a lack of sharp bend in the wire.
Even in the case of the kink 3b which is rounded as described above, such a round kink would not cause any problem when the distance between the edge 2b of the semiconductor chip 2 shown in FIG. 6 and the second bonding point G is as long as, for instance, 500 microns. However, in cases where the distance between the edge 2c of the semiconductor chip 2 and the second bonding point G is short, for instance, 300 microns or less, as indicated by the dotted line, then the wire 3 contacts the edge 2c of the semiconductor ship 2.